Folded cavity antenna

ABSTRACT

An antenna which conforms to the surface of a spacecraft can be a thin slot antenna which can be formed to the shape of the craft in combination with a compact ground plane formed of T-shaped or L-shaped pieces.

FIELD OF THE INVENTION

This invention pertains to a microwave antenna, and in particular to acompact antenna which can be made to conform to a cylindrical body in aspacecraft.

BACKGROUND OF THE INVENTION

Antennas in a spacecraft are used to telemeter information on spacecraftoperation to a ground station. Although external antennas can be used inouter space, only antennas integral with the skin of the spacecraft areusable during launch and re-entry. It then becomes a problem of sometechnical difficulty to design efficient antennas for useful frequencieswhich are light and not protruding from the spacecraft.

U.S. Pat. No. 3,172,112 to Seeley discloses a dumbell-loaded folded slotantenna. U.S. Pat. No. 3,346,865 to Jones discloses a simple slotantenna built into a dielectric radome. U.S. Pat. No. 3,518,685 to Jonesdiscloses a dielectric-loaded simple cavity antenna in a ballisticprojectile. U.S. Pat. No. 3,987,453 to Lopez discloses a balancedexciter for a wideband antenna element. U.S. Pat. No 4,197,545 toFavaloro et al discloses a stripline slot antenna adapted for use onaircraft or other vehicles. U.S. Pat. No. 4,367,475 to Schiavonediscloses a linearly polarized radiating slot antenna. U.S. Pat. No.4,373,162 to Peterson discloses a low frequency electronically steerablecylindrical slot array radar antenna. U.S. Pat. No. 4,460,894 to Robinet al discloses a laterally isolated microstrip antenna. U.S. Pat. No.Re. 29,296 to Krutsinger et al discloses a dual slot microstrip antennadevice. All of the above relate the same general area of technology, andall are different because choices of frequencies, size of the vehicleand environmental conditions make unique problems requiring differentsolutions.

A discrete antenna of a given size may have a beamwidth that results inless than optimal coverage in a two antenna system on a cylindricalgroundplane. Also a cavity antenna has an aperture that is too large tofit into a gap 2 inches wide.

A slot type antenna requires a ground plane of certain minimal size toradiate effectively. The minimum size required is an inverse function offrequency, making it difficult to accommodate the required ground planein low frequency antennas. A slot antenna operating at ultra-highfrequencies (UHF) would usually require a conducting ground plane 9inches on either side of the slot.

OBJECTIVES OF THE INVENTION

It is therefore a primary objective of the present invention to providemaximum spherical coverage of a discrete than antenna system formed on acylindrical body.

SUMMARY OF THE INVENTION

This objective of the invention and other objectives, features andadvantages to become apparent as the specification progresses areaccomplished by the invention according to which, briefly stated, a thinslot antenna which can be formed to the shape of the craft operates atUHF frequencies in combination with a compact ground plane formed ofT-shaped or L-shaped pieces.

List of Advantages of the Invention

An important advantage of the present invention is that beamwidth andfrequency can be adjusted independently of antenna cavity size.

A further advantage is that the antenna of the invention can be madevery thin, approximately 0.4 inches, for an antenna operating at UHFfrequencies.

Another advantage is that the antenna can be shaped to conform to thecurvature of the skin of a vehicle.

Still another advantage is that use of air dielectric eliminates theneed for tightly controlled dielectric material.

A still further advantage is that T-shaped or L-shaped groundplanesections reduce the size of the groundplane needed to operate a slotantenna.

These and further objectives, constructional and operationalcharacteristics, and advantages of the invention will no doubt be moreevident to those skilled in the art from the detailed description givenhereinafter with reference to the figures of the accompanying drawingswhich illustrate a preferred embodiment by way of non-limiting example.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a plan view of the antenna of the invention.

FIG. 2 shows an end view of the antenna of FIG. 1.

FIG. 3 shows a detailed view of the slot of the antenna of FIG. 1.

FIG. 4 is a sectional view along the section line 4--4 in FIG. 1.

FIG. 5 is a sectional view along the section line 5--5 in FIG. 1.

FIG. 6 is a plan view of the T-shaped ground planes.

FIG. 7 is a plan view of the L-shaped ground planes.

GLOSSARY

The following is a glossary of elements and structural members asreferenced and employed in the present invention.

11--antenna

13--upper copper-clad surface

15--lower copper-clad surface

17--slot aperture

19--honeycomb core

21, 23--Kevlar facesheets

25, 27, 29, 31--copper-clad edges

33--coaxial connector

35, 37--u-shaped elements

39--slot

41, 44--T-structures of a compact ground plane

45, 46--L-structures of a compact ground plane

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings wherein like reference numerals are usedto designate like or corresponding parts throughout the various figuresthereof, there is shown in FIGS. 1 through 5 an antenna 11 consisting ofupper copper-clad surface 13, lower copper-clad surface 15, and a slotaperture 17. The overall dimensions of the antenna are about 0.29λ inwidth, and 0.63λ in length, where λ is the free-space wavelength at thedesired operating frequency. The aperture 17 has the configuration anddimensions shown in FIG. 3, and is etched completely through the uppercopper-clad surface 13. The internal structure of the antenna 11 isshown in FIG. 4, and includes a phenolic honeycomb core 19, two Kevlarfacesheets 21 and 23, copper-clad edges 25, 27, 29 and 31. Materialsother than phenolic honeycomb and Kevlar may be used. The honeycomb maybe replaced by any low-loss, low dielectric spacer, such as polystyrenefoam. The Kevlar facesheets are structural and may be replaced by alow-loss, moderate dielectric material, such as Kapton, Delrin orTeflon-glass. The outer conductor of a coaxial connector 33 is connectorto lower copper-clad surface 15; the center conductor of the coaxialconnector is connected to upper copper-clad surface 13. The shape of theaperture 17 is unique and comprises u-shaped elements 35, 37 that areinterconnected by slot 39. This aperture radiates in an outwarddirection normal to outer copper-clad layer 13 and is shielded fromradiating inwardly by lower copper-clad surface 15. A major advantage ofthis antenna is that it may be tuned to particular frequencies bychanging only the aperture dimensions, primarily the mension "L" in FIG.3, and leaving the overall antenna dimensions unchanged.

In FIG. 6, there is shown a slot-type antenna 11 used with a compactground plane consisting of a conductive material such as copper oraluminum shaped into two T-structures 41, 44. The T-structures 41, 44are centered over slot aperture 17. The branches of the T-structures 41,44 are separated by no more than 0.04λ from the edge of the slot antenna11. The overall protrusion of the compact ground plane beyond the edgeof the slot antenna 11 is no more than 0.08λ. The width of the branchesof the T-structure 41, 44 are no more than 0.04λ. The length, L, of theT-structure is approximately 0.3λ and can be adjusted for optimalperformance at a particular frequency.

If space restriction requires, part of the T-structures 41,44 may beeliminated resulting in L-structures 45, 46 shown in FIG. 7. Theresulting L-structure 45, 46 will operate with only minimal degradationcompared to the T-structures 41, 44.

A major advantage of the compact ground plane is that it permits theintegration of a low frequency antenna in a complex structure withoutinterfering with that structure.

This invention is not limited to the preferred embodiment andalternatives heretofore described, to which variations and improvementsmay be made, without departing from the scope of protection of thepresent patent and true spirit of the invention, the characteristics ofwhich are summarized in the following claims.

What is claimed is:
 1. An antenna for use in the very-high andultra-high frequency bands, comprising:a first surface of conductivematerial, a second surface of conductive material equally spaced fromsaid first surface and closer to a cylindrical axis of symmetry, edgesof conductive material connecting outer perimeters of said first andsecond surface, and a slot means for radiating cut into said firstsurface.
 2. The antenna of claim 1 including a compact ground planeformed of two T-shaped structures.
 3. The antenna of claim 1 including acompact ground plane formed of two L-shaped structures.